The numbers we find in astronomy books describe the size of space precisely, but they don’t give us a good sense of perspective. Even the neighbourhood of our own solar system requires large numbers that we quickly lose our grip on.
The Moon, our closest neighbouring celestial body, is almost 400 000 km away. That corresponds to the distance a passenger plane would cover in 18 days, if it could fly without stopping. A journey to the Sun with such a plane would take about 20 years (if it could travel in a vacuum). That distance is a (astronomically) modest 150 000 000 km. And Neptune, the outermost known planet in the solar system, is 30 times farther away. But this is just trivial stuff in the bigger picture. Proxima Centauri, our closest star (apart from the Sun), is 40 000 000 000 000 away.
The numbers become so large that they are impractical. That is why astronomers have introduced the light-year as a unit of distance. One light-year corresponds to the distance light travels in one year. For comparison, light takes a little over one second to travel from Earth to the Moon. Using this unit, Proxima Centauri’s distance becomes 4.25 light-years. Distances like this are inconceivable. And the universe is much, much larger than that.
We actually don’t know how big it is. The only part we can relate to is what our telescopes can see, and so astronomers prefer to talk about the size of the observable universe. To gain at least a vague sense of this gigantic perspective, we need to use models. That’s where "The Planetfield" helps us.